Method for bonding resistors in electric furnaces



Jan. 12, 1954 A. .1. LINCOLN METHOD FOR BONDING RESISTORS IN ELECTRIC FURNACES Filed April 17, 1951 INVENTOR.

Patented Jan. 12, 1954 METHOD FOR BONDING RESISTORS IN ELECTRIC FURNACES Arnold J. Lincoln, Florham Park, N. J., assigner to Baker & Co., Inc., Newark, N. J., a corporation of New Jersey Application April 17, 1951, Serial N0. 221,525

2 Claims.

This invention relates to electric furnaces and more particularly to resistance heated furnaces of the type disclosed, for example, in Letters Patent No. 2,063,274, issued December 8, 1936, and No. 2,145,324, issued January 31, 1939.

Electric furnaces of such type comprise a heating chamber mounted in a housing with an electrical heating unit in the Walls of the chamber and insulating material filling the space in the housing around the heating chamber. In order to obtain the high operating temperatures that are required of these furnaces, the resistor of the heating unit is made to have a small crosssectional area and is embedded in a suitable refractory cement, such as alumina, and the like, for supporting the delicate resistor on the outside surfaces of the walls of the heating chamber. The resistor may be made of any suitable metal adapted to form an electric resistor, although wire or strips of the platinum metals (i. e. Pd, Pt, Os, Ru, Ir, Rh) and alloys thereof are preferred for this purpose.

It is a general object of this invention to increase the useful life of heating chambers and particularly of furnaces of this type. Other objects and advantages of the invention will become apparent in the following, detailed description, the appended claims and the drawing, in which the figure represents a mufe tube furnace construction to which the method of the invention may be applied.

Referring to the drawing, a conventional muiile type furnace is shown,y whichincludes a mule tube l which may be made of aluminum oxide, having a resistance wire 2 wound thereon and an outer covering 3 which may be of aluminum oxide cement.

The invention is based on the discovery that the useful life of an electric furnace of the type referred to above is greatly increased by subjecting the furnace to an initial super heat treatment before using the furnace for the purposes for which it was designed. This treatment is preferably performed at the factory where the furnace is manufactured and may be done in any suitable manner as by applying a high temperature ame or by passing an electric current through the resistor.

Furnaces of the type herein contemplated are used by dentists, laboratory workers, jewelry manufacturers, and others, for various purposes such as baking or glazing porcelain teeth, melting metals, annealing and so forth.

In the treatment of this invention the furnace is heated to an elevated temperature higher than used during the subsequent normal operation of the furnace and is maintained at this elevated temperature for a period of about two hours. The great improvement that results from this treatment is illustrated in the following comparative tests that are listed below.

Life of platinum wound furnaces run at 1500 C.

Furnaces Prclieated for 2 Hrs. at

reos 04,550 o. before rest I mees 0' Treated Six furnace mufe's were assembled in an exactly similar manner as` to resistor winding material, cemented with alumina, and dried. All muffles were wound with C. P. platinum Wire 0.020 inch diameter. Three of the furnaces were preheated to a temperature of from 1600 to 1650 C. and were Aheld at this temperature range for two hours. They were then allowed to cool to room temperature. All of the furnaces were then brought up under normal conditions to 1500 C. and were maintained at this temperature until failure occurred.

The treated furnaces showed an average of approximately 400% in increased life over furnaces not treated. After failure of the furnaces the alumina of the muiiles which had received the preheating treatment was very hard and the winding could not be easily stripped off from it. In those furnaces that were not treated the winding couldbe stripped from the alumina with relative ease after the furnaces had failed.

The furnace winding operates at a temperature of about 40 C. to 50 C. higher than that produced in the furnace. At the temperature involved in the treatment of this invention the alumina becomes semi-plastic whereby a very tight bond is made between the resistor wire and the alumina. Furthermore, the preheating drives the entrapped gases from the vicinity of the resistor winding thereby reducing the rate of volatilizaton of the Wire.

Refractories other than fused alumina may be used for cementing the resistor as long as they become semi-plastic at a temperature lower than the melting point of the resistor, e. g. ceramic materials. Other metals besides platinum metals and alloys thereof, that may be bonded to these refractories according to the invention are Nichrome, molybdenum and tungsten. When it is desired to bond metals other than the platinum metals to such refractory materials, temperatures different from those specified hereinabove are used. For example, Nichrome does not withstand temperatures in the range of 1600 C. to 1650 C. and may be bonded and embedded in ceramic materials, Molybdenum and tungsten resistors with fused magnesia, fused zirconia or fused thoria, when heated to temperatures of from 1700 C. to 2000 C. give results similar to those obtained when platinum and alumina are treated in the manner stated above. For the purposes of this invention the bonding temperatures are such that the refractory material becomes pasty, the entrapped gases around the metal are driven from the vicinity of the metal, and the refractory adheres rmly to the metal. In all cases the bonding temperature is maintained below the melting point of the metal. 1

The embedding of the resistor as hereinbefore described includes the complete covering of the resistor windings by the refractory material.

The fact that the semi-plastic state of the refractory material enables the driving off of the entrapped gases and provides a very tight bond is not only advantageous with respect to long life, but, when the wire winding is embedded as above set forth, also insures the stabilizing of the resistance characteristics of the wire, which otherwise changes with the prolonged use of the furnace so that caution must be practiced with respect to the current supplied to the wire. In other words when entrapped air is present adjacent the Wire, volatilization from the wire will take place upon heating with consequent reduction in the wire diameter and changing of resistance characteristics which brings about ultimate failure of the wire, whereas by following the method of this invention this occurrence is substantially prevented.

Havingthus described a specific application of this invention it is thought that the features and advantages thereof will be readily apparent to those skilled in the art.

However, the uses of the present invention are very numerous and there are many locations other than in a furnace where a heating chamber is employed and wherein the method taught by this invention is advantageous in prolonging useful life of the apparatus.

The foregoing disclosure is to be regarded as descriptive and illustrative only, and not as restrictive or limitative of the invention in any manner other than by the scope of the appended claims.

What I claim is:

1. In the manufacture of resistance type heating chambers, a method for increasing the useful life of such chambers wherein an electrical heater element of a metal selected from the group consisting of the platinum metals is embedded into an alumina refractory material, comprising the steps of assembling the heater element and refractory material into a desired heating chamber, heating the assembled combination to a temperature within the range of 1600 C. to 1650 C. at which the said material softens into a paste-like mass but above the subsequent operating temperature, whereby any gas entrapped adjacent the heater element is driven out and a tight bond is provided between the resistor and the material and maintaining said temperature above the operating temperature for about two hours prior to employing said chamber for its intended usage.

2. In the manufacture of electric furnaces the method for increasing the useful life of the heating chamber thereof comprising the steps of winding a resistor element of a metal selected from the group consisting of the platinum metals around a mulfle member, covering said resistor element with a layer of refractory material, assembling the mufe, resistor and refractory material into a furnace organization, heating at least the refractory layer to a temperature within the range of l609 C. to 1650 C. at which the said material softens into a paste-like mass but above the subsequent operating temperature to drive out any gas entrapped adjacent the winding and to obtain a tight bond between the winding muiiie and refractory material, and maintaining said temperature above the operating temperature for about two hours prior to employing said chamber for its intended usage.

ARNOLD J. LINCOLN.

References Cited in the le of this patent UNITED STATES PATENTS Number Name Date 1,158,971 Boeck Nov. 2, 1915 1,158,972 Boeck Nov. 2, 1915 1,164,840 Miller Dec. 2, 1915 1,402,832 Brown Jan. 10, 1922 1,533,288 Woodson Apr. 14, 1925 1,842,433 Terwilliger Jan. 26, 1932 2,063,274 Stauss et al. Dec. 8, 1936 2,089,541 Dallenbach Aug. 10, 1937 2,145,323 Stauss et al. Jan. 31, 1939 2,195,297 Engle Mar. 26, 1940 2,355,443 Jeffery Aug. 8, 1944 2,500,305 Ackley Mar. 14, 1950 2,585,791 Klein Feb. 12, 1952 

